1. Field of the Invention
The present invention relates to an improved plasma source arrangement for providing ions for implantation into an object. The invention is especially suited for hardening by ion implantation of large objects such as die tooling materials for the automotive industry.
2. Description of the Related Art
Plasma source ion implantation (PSII) is an ion implantation technique which circumvents the line-of-sight restriction inherent in conventional ion implantation. The basic technique is disclosed in U.S. Pat. No. 4,764,394, entitled "METHOD AND APPARATUS FOR PLASMA SOURCE ION IMPLANTATION", issued Aug. 16, 1988, to J. Conrad (University of Wisconsin Alumni Association). Ion implantation into surfaces of three-dimensional target objects is achieved by forming a plasma about the target within an enclosing container and applying repetitive pulses of negative high voltage between the target and the conductive walls of the container. Ions from the plasma are driven into the target object surface from all sides simultaneously and omnidirectionally without the need for manipulation of the target object. The plasma may be formed of a neutral gas introduced into the evacuated container and ionized therein with ionizing radiation so that a constant source of plasma is provided which surrounds the target object during the implantation process. Significant increases in the surface hardness and wear resistance characteristics of various materials are obtained with ion implantation using the PSII technique.
The conventional PSII plasma-production process disclosed in the Conrad patent is limited in several aspects, which severely restricts it application to ion implantation of large-scale objects such as dies and tools used in manufacturing automobiles. First, direct ionization of gas atoms in the PSII working volume is difficult and impractical to implement for producing large-scale plasmas, generally larger than one square meter, with densities between 10.sup.6 to 10.sup.11 ions/cm.sup.-3, required to surround large scale dies for ion implantation. For filament discharges inside the container, the filament must be heated to a high temperature, on the order of 2000.degree. C., for sufficient electron emission. At this temperature, evaporation of the filament material occurs which would be undesirable since it would coat the tool or die part without complicated baffling. Radiation emitted from unbaffled hot filaments would heat the die surface. For large scale plasmas, on the order of 20 m.sup.3, a large number of filaments would be required, on the order of 20 filaments, producing as much as 1 W/cm.sup.2 of heating radiation power on a die surface of 1000 cm.sup.2 in area. For a 1 mA/cm.sup.2 plasma implantation current density, 100 kV implantation voltage, and 1% duty cycle operation of the pulse modulator, the average power density heating the of die surface by implantation alone is also 1 W/cm.sup.2. Therefore, if a conventional filament discharge technique is used for large scale plasma generation, it will double the power heating the tool or die surface during implantation, with potentially deleterious effects.
Where more than one ion species is desired for implantation, as in the case of multi-ion implantation, the ion-production of each ion specie cannot be selectively controlled when the ion generation occurs in the same container which encloses the target object. This is because all gas atoms become ionized collectively, rather than selectively. Thus, the basic PSII technique as disclosed by Conrad is only suitable for implantation of a single ion species.
In PSII, it is important to produce a plasma that is uniform in density, in order to achieve uniform implantation. In any plasma production process, plasma density gradients are always highest near the plasma production or generating source. In a conventional PSII apparatus, this restricts the location of the plasma source in the container which encloses the target object, and places yet another highly limitative constraint on the configuration of the apparatus.